Signaling system



June l0, 1930. H. s. "BLACK I SIGNALING SYSTEM FileqJnn ze, 1928 zsheets-sheet 1 A rTH/VEY June 10. 1930. H. s. BLACK i SIGNALING SYSTEM Filed June26, 1928 2 Sheets-Sheet 2 Patented June 10, 1.930

PATENT olricla'.l

HAROLD S. RLACK, OIF EAST ORANGE, NEW JERSEY, ASSGNOR'TO: BELL TELEPHONE LABORATORIES,' INCORPORATED, OF NEW YORK, N. Y., A' CORPORATION OF NEW YORK Application med June 26,

This invention relates to Wave transmission systems, and especially to tWo-Way transmission systems involving one-Way amplifying devices. i I

An object of the invention is. to facilitate reduction of wave reflections in such systems.

An example of applications ofthe invention is found in reducing crosstalk that, as explained in H. S. Black Patent 1,676,627, July 10, 1928,- arises in a carrier Wavesignaling system because a. repeater or-a terminal circuit in the system reflectscros'stalk currents created in the system by a neighboring system, and the reduction of crosstalk that, as explained inthe copending application just mentioned, is created in the neighboring system because the repeater or the terminal cir` cuit reflects signal currents that it should pass. i 2o For instance, a carrier .telephone repeater system of a general type disclosedjn U. S. Patent to K. S.,Johnson and M. B. Long, 1.606,795, Novei'nber'A 16, 1926 or in U. S.

1,7, 1929, maybe considered. In sucha sys/ second frequency range, below the first range.

They pass through an input lovv pass direction filter (or low frequency transmissionv band filter), thence through an attenuation equalizer Which eqn-alizes the attenuation of the secondline section (and certain'apparatus associate-d with it) over the second frequency -range, thence 4through a unidirectionally transmitting amplifier,- and thence. through an output lon1 pass directionfilter to the first Patent to C. W7.- Green, 1,7 39,668, Decemberv or direction filter or highA frequency trans- Waves from the latter line section lie in 'a'- SIGNALING SYSTEM 192s.` seriai No. 288,355.

line section.

repeater. Thus Waves from the output circuit of` a repeater amplier are prevented from reaching the input circuit of the amphfier.,v In one specific aspect the present invention is a system of this general type but' Wit-h the equalizers disposed between portions of.the input filters, a major portion of each input filter being connected between an equalizer and an amplifier. An attenuating network, Which compensates .for lower attenuation of the second line section over the low frequency ran-ge than over the high frequency range, is also connected betweenv the two porf tions `of the low' pass input filter. Each equalizer is of a type which has its characteristicA impedance aV constant resistance for thev rangeof frequencies overwhich the equalizer is to equalize attenuation. A Such The four filters serve to separate theoppositely directed signals in the.

eqnalizers arel disclosed in U. S.A Patent to 1 O. J. Zobel, 1,603,305, .October 19, 1926.

An important advantage arises from thus locating .the equalizer that is in the high frequency path through the repeater at a position ahead'of the major portion of the input 'filter in that path, and loca-ting the equalizerand theattenuating network which are in that current reflections in their'epeater,` which produce crosstalll: asindicated'labove, will not be ktoog'reat.v AAs a. consequence, the design ofthe filter is made much easier and individual elements of the filter can be made more cheaply.

Preferably the unidirectionallyl transmitting amplifier thatamplifies the signals transmitted in one -direction over the line also amplifies the signals transmitted in the other direction, in the general vmanner disclosed in United States `Patent to J. S. Jammer, 1,527,651, February 24, 1925 orUnited States Patent to 'J."'S. Jammer Iand C. W. Grreen,

No.' 1,658,338, February 7, i928. A .feature equalizer4 in the high pass input filter equalizes transmission ,-of the signal currents transmitted in one direction over "the` line without affecting signal currents transmitted in the other direction; and the equalizer in the low pass filter .equalizes transmission'of the "si nal currents in' theother directionwithout a ecting signals r'transmitted in the first direction. Further, the equalizer may be adjustable for varying the gain of the ampli- .fier circuit by different amounts atdifi'erent frequencies, independently for the two directions of transmission over the line. The adjustments of the attenuation equalizing and i gainadjusting means may be madeby automatlc ,gain controllers', respectively respon-v vsive to pilot'waves of different frequencies transmitted tofthe repeater from the different directions and selected by filters after passing through the repeater amplifier.

Other objects and features' of the invention will be apparent from the following description and claims.

.Figs 1 andf2 are circuit diagrams of carrier telephone .systems embodying two different forms of the invention, respectively.

In Fig. l a unidirectionally transmitting .amplifier 1 transmits carrier signaling waves in 'both-directions between sections'l and 2 of a carrier transmission line which'transmits Voice frequency waves also. 'The carriersignaling waves from 'line sectio'n21and the voice. rfrequency waves from that line i l section are'separated by a [high pass filter 4,

and a low'passfilter 5 of a high'frequency composite set. Similarly the carrier signalingwaves from line section 3 andthe'voice frequency waves from lthat line section are separated. byfa high pass `filter 6 and a low pass filter 7 of a high frequency composite set. f

The carrier signaling waves may becarrier waves modulated by speechjor other modulating waves. The carrier signaling Awave sfrom 'line section 2 lie in onecarrier frequency range. They pass through a transformer 9, a small portion 10 of an input high pass filter (or high frequency transmission band filter), thence preferably through a resistance padA or1 attenuating network 11,

. thence through anattenuation equalizer 12 which equalizes the attenuation of line section 2 (and the filter 4 and the input 'high pass filter) over the one frequency range, thence through the major portion 13 of the input high pass filter, thence through the unidirectionally transmitting amplier 1, and thence through an output highpass direction filter 14, to the'line section-3. Thecarrier signaling waves from the latter line section lie'in a second carrier frequency range, below the first carrier frequency range. They pass through a transformer 19, a small portion 20 of an input low pass filter (orV low frequency vtransmission band filter), thence through a resistance pad or attenuating vnetwork 21, thence through' an attenuationequalizer 22 which `equalizes the attenuation of the line section 3 (and the filter 6 and the input low pass filter) over the second carrier frequency range, thence through the major portion 23 ofthe input lowpass filter, thence through the unidirectionally transmitting amplifier 1, and thence through an output low pass direction ,filter 24 to the line section 2. The' input high pass filter, the input low pass filter, the output high pass filter and the outputflow pass lter prevent waves from the output circuit ofthe repeaterV amplifier from reaching the input circuit of the amplifier. The equalizers 12 and 22 are disposed between portions of the input high soA pass filter and the input low pass filter rep spectively, a major portion 13 ofl the high` pass input filter being connected' betweenequalizer 12 and the input side of amplier 1,

and a major portion 23 o-f the low pass input filter being connected between equalizer22 and the input side of amplifier 1..4

Eachof the networks 11 and 21 is adjustable for Varying its `attenuation While maintaining the impedance which it presents atv its input terminals and at its output termi'- nals -a constant resistance impedance. The network 1 1 in conjunction with a variable equalizer 41 described below, adjusts the transmission equivalents ofthe channels of the high frequency range, to take account of different attenuation characteristics for dif- 1 ferent lines in which the repeater may be used. The maximum attenuation of net works 11 and 21may be, for instance, 2 TU and 12I TU-` respectively. The attenuation of network 21 is the larger because that nety work notonly serves, in conjunction with a variable equalizer 51 described below, the

same purpose with respect to the channels of the low frequency range. that has .been described for network 11 and equalizer 31 with respect to the channels of the high frequency range, but also compensates for the lower attenuation of line section 3 over the frequency range of the carriersignaling waves transmitted to the repeater from line section 3 than over thejfrequency range of the carrier signaling waves transmitted from the repeater to line section 3. 7 The networks 11 and 21 are connected between the portions of the input filters between which the equalizers 12 and v22, respectively are connected. As-

. will be made apparent hereinafter, is especially advantageous to have the 12 TU network 21 positioned'between the major portion of filter 23 and ltheline section 3.

The equalizer 12 has its impedance facing network 11 a constant resistanceimpedance, This equalizer comprises a basic or fixed attenuation equalizer 10 anda variable equalizer 41 connected in tandem. The fixed equalizverv 40 maybe forexainple of the type of the equalizer 'shown inlFig; 16 of 4Zobel Patent 1,603,305, October'19, 1926. The attenuation of the fixed equalizer 40 may be constant for any frequency but may decrease as the frequencyincreases, or in other words have an attenuation frequency characteristic of negative slope. `The variable equalizer 41 may be for example .anetwork such as the adjustable artificial line of Fig. 4 of Zobel Patentv 1,591,073, July 5,1926.- the variable equalizer-41 i's adjustable, and increases with frequencyor 'in other Words has a characteristic'of positive slope. By varying or adjusting the equalizer41, the gain of the repeater from line section 2 to line section can be varied at will independently of the gain for transmission in the opposite direction, and by such different Aamounts for different- 'frequencies as to compensate, over the range of frequencies transmitted through the high pass input filter, for variations ofattenuation vofthe line. section 2, which of course 'are different for different frequencies. The vvariations of attennation of the line section 3, over this frcquency ran ge or for .transmission in the direcJ tion from linesection 2 to line section 3, are

taken care of by an equalizing gain control' `transmission through line section 3, filter 6 and the 'low pass input filter instead of through line section 2, filter 4 and the high pass input filter. Consequentlyv` b v varying oradjusting` theequalizer 51 the gain ofthe repeater from line section 3 t-o line section 2 can be'vai'i'ed at will, independently `of thel gain' for transmission from line section 2 to line section 3, and by such different amounts foi' different frequencies as to compensate for' variations of attenuatioi'i of the line section 3, over theange'o'f frequencies transmitted through the lou7 passV input "filtei". The variations of attenuation of the line section .2,

The attenuation of for this frequencj7 range or .for transmission in the d'irectionfrom linesection 3 to line section 2', are taken care of by an equalizing gain control means (not shown) at that end of section 2 remote from the repeater shown. Thus,in`conjunction with that means, the equalizer 51 maintains a substantially constant overall transmission equivalent of the line vfor. transmission in the direction from section 3 to section 2, regardless of changes such for example as changes in temperature and humidity inthe weather.

lt'will be apparent that although the rc'- peater amplifier is common to the Vdpaths for transmission in opposite directions over the circuit not only the gain contiol but also the attenuation equalization is independently vadjustable for the twoy directions of transmission. 'The equalizing and gain control" means are of a type not adversely affected by the 'high poiver'level ofv the output wavesy from amplifier Il'in spite of the fact that the equalizing andgain control means get little protection (by filters 10 and 20) from those high power levels.

j As indicated above, undue current or wave reflections'from t-herepeatcn'an'd consequent crosstalk between' adjacent parallel carrier circuits (of which only one `circuit is shown), are lavoided by the positioning of the equal-v '.izer 12 ahead of the major portion l'ofthif input high pass filter as regards energy flow through that'filter, and bythe positioning of the equalizer 22 and the 12 TU attenuatiiig pad 21 ahead of the majorportion 23 of the. input lowpass filter, as regards energy flouv through that filter. Near end crosstalk can be prevented by having thc frequency y range ofthe carrier signaling currents the same I'for the two adjacent circuits, for each direction of'transmission. F or example, consider line section 2 and the corresponding section of the adjacent parallel circuit `which ma y beregarded as repeated at the same point as the circuit shown. Even if a signal modulated carrier Wave of high power level from the output of the amplifier (not shown) in the adjacent circuit, corresponding to amplifier 1, induces a wave in section 2 the induced ivave is. of such low frequency thatv it will not through the portion 13 of the input high pass filter shown. This frequency allocation likjeii'fise prevents objectionable near end crosstalkfrom line section 2 into thel adjacent circuit. Similarly it prevents near end crosstalk between line section 3 and the 'corresponding sect-ion of the adjacent paral-l lel circuit. i. i

However, if a signal modulated carrier -wave of' high power level from the out-put of the amplifier (not shown) in the' adjacentr circuit, cori'cspondingto amplifier 1, induced a Iwave' in section 2 and theindnccd wave encountered a considerable change in impedaiiceuponits arrivalat the input 'high passv filter of the repeater shown, then a considerable portion of the energy of theinduced wave would be reflected from the'inputhigh pass y pass through a filter (notshown) the waves transmitted from filter Y14 to line 3, the impedance seen looking into filter 20' should be high enough to avoid unduly shunting these waves away from the line. By way of example each of the filters and 20 1s v shownas consisting of oneand one half sections. As regards the inputhigh pass filter and the input low pass filter, reliance 1s '60 A`placed principally .on the filter portions 13 "filter 24: and be hearflas CrOSS'tll. Likewise; vand 23 for separation o'f the messages transif a considerable impedance irregularity eX-v nlitted in opposite directions over the circuit,

' isted at the input high passfilter (not shown); these filter pcfrtions preferably contaiiriug 'of .the repeater in the adjacent circuit, corre?r enoughfilter sections to produce substantially spending to repeater 1,.' that impedance irfeall of the attenuation required in the input regularity might occasion objectionable crOsS- filters for separating the different frequency talk in the absence of preventive means. 'groups of carrier signaling waves.. v

Similarly, any pronounced impedance mis- I If the filter portion 13 following the equalmatching at the input lowpass filter shown izer 12 has its impedance Within its frequency o at the corresponding input low pass filter range of free transmission and down to witholf the adjacent repeater of. thev adjacent cirin about 2,000 cycles of its 'cut-off frequency,

cuit, might resultin objectionable crosstalk ofthe propolvalvuo to .avoid production of between the carrier circuit shown and the adobjectionable refiections, then very litt-1e atjacent carrier circuit, inthe absence ofpre- Atention need be given to the impedance of the ventive means. filter portion outside of its transmission Ordinarily itis very difcultand eXpenband, since the equalizer loss becomes higher sive to prevent objectionable reflection in and higher-at the lower frequencies, andl any the input filters, of a carrier repeater, since waves 'reflected from this filter portion to'- even slight departures, from .the theoretically Ward the line. 2 must ,pass through t-he equalcorrect impedances of the elements of the izer 1.2 and' suffer attenuation in that equalfilter are-sufficient to cause aconsiderable 'reizer before reaching the line 2. As a result,

flection. However, when an equalizer of the the design ofthe input high pass filter is l'constant resistance impedance type, referred made much easier and the individual eleto above, follows a filter in a circuit, such forl ments of the filter can be made more cheaply.4

example'as filter 10, the equalizer largely de- In the neighborhood of the cut-ofi' freter'mmes the terminating impedance of the queucy of the low pass filter 2'3, Which neighfilter,r so that the design requirements for the borhood is a frequency range in which the network following the equalizer, such foreximpedance .of this filte'ris poor regards` l ample as ofthe network 13 are considerably matching the equalizer impedance that faces eased as to its impedance as part of the terit, the Aattenuation of equalizer 22 is lower .mmatmg impedance of the filter. Thus, for than at lower frequencies.I However, the 12- the major portions 13 and 23 of the input fil- 'TU pad 21 supplements the action of equal- `tersof the repeater shown, the design requireizer 22 in Vattenuating reflections from the ments lnecessary'to avoid. objectionable cur filter 23.l rent reflections .from the repeater and conse- The high quent crosstalk between parallel carrier cirflections which ,are cumulative Vwith refleccuits arevconsiderably eased by placing the tionsproduced in thelow pass filter 23.

equalizer 12 and 22 ahead of those portions, However, the equalizer 22, as well as the respectively. The resistance networks 11 and 12 TU pad, hasI high attenuation for the lat- 21, if present, also tend to render the impedter reflections at frequencies, in. the neighance requirements for'the filters 13 and 23, reborhood of or lower than the cut-off fies pectlvely less severe. Preferably the numquency lof the filter 6, which are frequencies ber'of filter sections positioned ahead of each 'at Which the vimpedance of thefilter 6 is a lpass linel filter 6 produces reof the equalizers 12 and 22, i. e., the number poor match -for the line impedance that it '1 of filter sections -in the portions 10vand 20 of faces. the mpu't high pass filterV and-the input low pass filter,respect1vely, 1s only sufficient to electrostatic shields 45, of copper for examsubstantiallymatch the impedances of the ple. between the primary and secondaryersI repeater and those of the-transformers 9 and windings. I The shielded transformers i-n- 19 for the high and low` frequency groups crease the attenuation of the circuit for lonof carrier frequencyfslgnalmg waves. For the gltudinal currents, and render it feasible to frequency Irange -of -the waves transmitted have the filters 1'0, 13, 23,20, 24 'and 14 of fromiilter 24 to line 2, the impedance seen 'the unbalanced tv )e as shown. Allf th looking into filter 10should be high enough filters, and the filtlers 'l, 5,'6, and?, aond to prevent unduly shuntingthese'waves away a filter 48 to be referred to presently may be from the line andfor thefrequency range of constructed in accordance with the principles The transformers 9 and 19 preferably have l disclosed. in ainpbell Pateiit1,227,113, May 22, i917-, and 1,493,600, May. 1a, 1924,.-and Zobel Patent 1,538,964, May26, 1925.

By switches 46 a low. pass filter 48 may be connected in circuit between pad 11 andA equalizer 12, if desired. The cut-off' vfrequency of this filter is preferably just above ,signals transmitted over the circuit.` Thus. .sincev the filters 4 and'6 preferably have al the upper limit of the frequency range 'of the cut-off"frequencyv just beloviT the lower liniivt o-f the carrier signalstransmit'ted over the circuit, the repeater-'amplifies only wavesvof frequency withintlie frequency .'range ofthe carrier signaling Waves transmitted over the. circuit.` The filter` 48 prevents overloadingv of the-repeater by interference from power line carrier waves'. v power linecarrier signaling systems ordinarily areofl highv power relative to the power .of carrier signalingwaves in carrier telel,

'- phone transmission lines and lieinaff're- -quency range `somewhat above the `frequency .range of Ythe'latter waves. For exam/ple, t-he upper limit of the carrier signalingy requen-l cies transmitted by the repeateriiiaybe 3 0' kiloc'ycles whereas the power y'line' carrier signaling waves may lie 4in the frequency range between 30 kilocycles and 40 kilocycles.

If desired, instead of including the low pass high cut-'off' filter V48 in circuit, the purpose served by the inclusion of that filter may be accomplished .by making thefilter 4 or the' filter 13 aband pass filter with its upper z cut-off frequency equal'fo that which the filter 48 would have' if the 'latterfiltei' were used.

If desired fthe adjustment of the variable `equalizers 41 and. 5,1 respectively can be made automatically, by two motors respectively, under the" control of two pilot frequency waves of two' different-frequencies l sent to the repeater over lines 2 and 3 re y spe'ctively and selected by two tunedfcircuits j 0r frequency selective means 28 respectively after amplification in amplifier 1. 'The pilot frequencies may lie in the frequency range vbetween that of the group of signal side band components transmitted to soy tlie repeater over line 2 and that of the group of signa-l side' band components transmitted The filters 10, 13 andl 14`may -be made to include in their' to the repeater over lin'el pass ranges .thefrequency of the pilot wave transmitted over line 2"but not the other pilot frequency; and the filters 20, 23 and 24 ina be madeto include iii their )ass v .Y l

i'ii its output circuit. 'controls' the niotor. controller 33( foi' Athe The carrier Waves'in wave. Tlie-reference characters 28, 30, 31,

32, 35 'and 3 6 designate respectively the saine elements as iiivthe'Affel patent. The reference character 33" designates the motor rcversing switches and power ysupply (not shownlwhich are designated 33, 34 and 38 in the. Affel patent. Resistance :30 prevents all n of the energy of the pilot frequency to which `1 .the tuned-circuit 28'is selective from 4being v I absorbed atl the first repeater or automatic gain 'control means encountered. frequency from the tuned circuit 28 is fed to a circuit comprising an amplifier A and' a The pilot detector D whichhas acontact inillia'mmeter The inillianimeter motor 35. y

Lplui'alityof switches 36 are driven by the motor 35, astheswitch 36 of' Affel is driven'v by motor 35 of Aifel. Eachof-these switches is individual to one .ofthe attenuation adjusting resistancessuch as R12 and liriof the variable equalizer (wliich.corre' spond to resistances R12 and Rg., of Fig;1 3 of the above mentioned Zobel-Patent 1,591,073).

l/Vitli `the transmission line infnormal coiidition, the pilot frequency wave transmit-ted over the line will be of sufficient amplitude to maintain the contact of milliammeter32 in theine'utral position. When the amplitude becomes'grea'ter orless, operation of the milli ammeter in one direction or the other results in the motor controller 33 causingniotor 3.5

to operate clockwiser orl counter-clockwise. respectively. Consequently, switches 36 are driven to adjust the resistancesl1L andfRg.,I :to again bring-.the amplitude of the pilot frequency wave in the input circuit of amplifier A to the'nornial valine. The number of.

pairs of switches driven by the motor 35 is the same as the number'of sections and hal-f sections of the variable equalizer th'at are to be varied in theirattenuation setting. If each ofthe resistances in the equalizer is so varied, then, as explained in Zobel 1,591,073,

y mentioned above,4 l the attenuation of the equalizer can be varied considerably without producing undue departure from the ideal .attenuation-frequency characteristic for the variable equalizer.

If lgreater `variation ofthe `attenuation is desired, then instead of having the gain controlling iiiotor 35 vai-y the settings of the viesistaiicesfof the individual sections o f the variable equalizer,means (not shown) can be provided 'whereby the motor 35 operates switches that connect any desired number 0f halfl sections of the variable equalizer in circuit as shown iii 4ie'. 4 ofthe above mentioned Zobe'l Patent 1,591,073.

F ig. 2 shows a systein'whicliis like that of Fig. 1 except that no automatic gain controller is shown and instead of having a Coinn'ion amplifier foi' the two directions of transmission separate amplifiers 1E and UV are used forthe two directions in the general manner disclosed in the above mentioned pat.

ent to K. S.Johnsor and M. B. Long. The operation of this lsystem will b e apparent from the description of Fig. l given above,

What is'. claimed is: y h

l'. A wave transmission system'conil'irising a device for controlling the transmission equivalent of the system for waves to be. transmitted through the system, said device having an input path and an output path, means for. applying'waves of two different frequencies 'to said input path, the intensity of the Waves of one frequency having a lower order-of magnitude than .the intensity. of the waves of the other' frequency, andv frequency selective means connected to said output path of said'device, for passing said one frequency.Y

to theexclusion of the other, said device transmitting both of said frequencies with attenuation low compared to the attenuation,

" of said frequency selective means for said other frequency.

2. A wave transmission system comprising a device for controlling the transmission.

equivalent of the system for waves .to be transmitted through. the system, said device having an input'path and an output path,

means for applying Waves of different fie-' lquencies to saidinput pathiof saiddevice, the

intensity of the waves of one frequency having a nigher order of\magnitude than the intensity of the waves of another frequency and said device transmitting both of said frequencies, an electric space discharge' devicehaving an inputpath, and frequency selective means, connecting said output path of said first device to said input-path of said dischar e device, for passing-said other freuency ut pir-ventingr the waves of said one requency received from .said first means from reaching said input path of said dis.

`charge device.

3. A wave transmission system comprising a line, means for equalizing the attenuation of said line at different frequencies, said means having an input path and an output path, an electric space discharge device have ing an input path, frequency selective means connecting said output path of said means to said inputpath of said device, and means, in-

cluding saidrst means and a portion of said line, for applying to said frequency selective circuit waves of one frequency which have intensity of one order of magnitude and` Waves of another frequency Which'have in.- .teiisity of a higher order of magnitude, saidfrequency selective means passing said waves -of said one frequency to the exclusion of saidwaves of said other frequency.

4. A' Wavetransniission system comprising a line; means foi" equalizing the attenua'- tion of said line at' different frequencies, said means having an input path and an output .path and having its input impedance a conof said device.

stant resistance for said frequencies, a circuit connecting said line to said input path, an

electric space discharge devicevhaving an input path, and 'a wave filter connecting said output path of said means to said input path 5, The combination with a lineof a twoway repeatertherefor, said repeater coniprisl stant resistance for said frequencies, a circuit connecting said line to said input path, and a wave filter connecting said output path of said means to said input pathof said device. 6. The combination of a transmission line,

.means for equalizing the attenuation of said line at different frequencies, said means having/ an input circuit 'and an output circuit,

an amplifier-having aninput circuit and an output'circuit, means-including said output circuit of'said amplifier and a portion of said Vline for applying` waves ,of dierent `fre- `quencies to said input circuit `of saidequalizing means, the intensity of said Waves of. one frequency having a diHerent order of magnitu'de than the intensity of said-waves of v another frequency, and a frequency selective circuit, connectingsaid output circuit of said equalizing means to said input circuit of said amplifier, for passing said waves of said one frequencybut vpreventing said waves of said other frequency received from said means fiom reaching' said input circuit of said amplifier.` 7. In combination, a" linc, acii'cuit for transmitting Waves to said line, a circuit for transmitting waves from-said line, a oneway path connecting saidcircuits, and an attenuation equalizer in one of said circuits forv a range lof frequencies.

8. The combination with two lines, of two channels interconnecting said Lines, said two chann'elscoinprising a common unidirection- `ally transmitting portion, a portionindividual to one of said vchannels for-transmitting waves from one of said lines to said com mon portion, a portion individual to said other channel for"transmittingwaves from said other line to said common portion, means in said first individual portion for equalizing the attenuation of'. said one line over a range `e'quali'zi'ng the atteniiationvof said line vover izo portion, a passive ii'npedance network connecting one of said lines to said common portion, a passive impedance network connecting -said other line to said common portion, means in one of said networks for equalizing the attenuation' of one-.of saidlinesover a range of'frequencies, and means in said'otheifnetwork for equalizing the-attenuation offs'aid. other lin'e over another range of frequencies.'

V 10.-' Two lines, an amplifier for, dimes,

path for transmit-ting waves the ffii'st said line through the amplifier i ,given d-i- .ilrection...tn themnherline, another'l/path for .transmitting -waves lfroinQ/sa'id .other 'liner through said aniplier iii-said direction to said rangeof frequencies.' c i 1l.vv A wave transmission system compris- -ing two lines, acne-way `amplifierfor repeating from one of .said llines to theother and from said other line to said one line, output frequency grouping direction filters .con-

' ne'cting the output side ofsaid amplifier to said- ,linesI respectively, frequency selective paths respectively. coiinecting said 'v lines to the input side" of said amplifier, frequency'l grouping direction""filters'. included 'insaid l paths respectively, and line attenuation equalizers'for said lines respectively, connected between portions of said filters respectively and between major portions ofsaid filters 'and said lines respectively, each of said equalizers havingits input impedance a constant 'resist- Vance for the frequencies to .be therethrough.

12. A'wave transmission system comprisii'ig a wave attenuating device adjustable for varying its attenuation by different amounts vat diffe-rent frequencies to be' transmitted through the system,'said deviceliaviiio' an input path and anoutput path, means yIor apv pl'ying waves of two different frequencies to said input path', the inteusityof the waves of one frequency having a lower order of niagnitude-tlian the intensity of the waves of the other frequency, and frequency selective means connected vto said output path of said.

device, for passing said one frequency to the exclusion ofthe other, said device transmitting both ofsaid frequencies with attenuationv low compared tothe attenuation of .said frequency selective means vfor said other frcquency. Y v

13. In combination, a line, a. frequency selective 4circuit for transmitting waves of a given frequency rangeto said line, a frequency selective circuit for transmitting waves of another frequency range from said line` a one-way path connecting said circuits, and wave attenuating means in one of said circuits,.adjustable .for varying its attenu- 'ferent frequencies in opposite directions be:

transmitted ation by different amounts at different frequencies transmitted by said one circuit. y

14. In a signaling system, the combination with two lines for transmitting. current, of

two channelslfor transmitting currents of diftween 'saidliiies, said ltwo channels comprising a common'unidirectionally7 transmitting 1y. 1 portion, a,passive"impedance network con necti-ng one of saidlincs to said common pn tion, a 'passive impedance networkvconnect ing said other line to said common portion,

., wave -attenuatii'ig means in` one ofv said -networks, adjustable for varying its attenuation by'different amounts at different frequencies lover' ,a range of frequencies transn'iitted by.

that network, andv'ave attenuating means Vin said other network,adjustable for varying vits attenuation diEerent/,amounts at different frequencies over a range of frequencies transmitted by-that network.

l 15.. A two-way repeater, for a waive transmission system comprising'two hiiersect :ns

vfor two-way transmision'in which-thefreequenciestransmitted in opposite directions are comprised in different frequency groups, s-aid repeater having input and output lbranches for connection to each o f'said line sections,.a.coniinon amplifier for all of said waves connected between said branches,` two,-

frequency grouping direction lfilters `in said j,

output branch', two frequency grouping` input ,filters in said input branch, one at each side of the junction of saidv amplifier with said input` branch, an attenuation network connected between two portions of one of" the two latter filters, adjustableito vary its attenuation by different predetermined amounts for different frequencies in la frequency range passed bythe filter in which it is connected,

an attenuation. network connected between quencies. in a-v frequency range passed; by sai d other filter, a major portion of each' of said input filters being connected between said junction and said networks, and each of said networks presenting to the other portion of onev of said filters an `impedance which is substantially a vconstant resistance over the frequency Arange ofthe waves'to be trans' mitted through the filter;

16'. A wave transmission-system compris-` ing` a line, meansv for equalizing Athe attenuation of' said-line over one group of frequencies, said means having an input path and an output path, an electric space discharge device having an inputpath, a circuit including frequency selective means for passing said one group of frequencies and pre-. venting passage of a higher group of frequencies, said circuit connecting said output path of said means to said input path of said device, means, including said first means and said line, for applying to said circuit waves of said higher -frequency group which have intensity of a higher order of magnitude, and

other input pathf-for passing Said lower range and suppressing said' one range, an attenuation equalizer"l connected between said one line and a major portion of said one in-4 a-wave attenuating pad connected in said cir- /p'ut filter, another attenuation t equalizer and -cuit between said equalizing mejans and said a transmission equivalent adjusting network lfrequency selective means. coniiectedzbetween said other lii'i'e and a ina-` 17. .A'wave transmission system comprisjor portion of'saidother input filter,.sa1d one ving two lines, a one-Way amplifier for repeatf equalizer vequalizing the Vattenuation of said ing from o'ne of said lines to the other and one line over said vone frequency range, `said fromsaid other line to said one line, one inother equalizeruequalizing the attenuation of put path connecting said one line to the input said' other -line over Asaid lower frequency side of said amplifier, another input path range, said network compensating for lower connecting saidother nliiietp the input side" attenuation ofsaid other line for said lower of said amplifier, one 'output pathconnecting- Afrequency range than for said one frequency the output side of said amplifier to said other range,l and veachl of sai-d input paths- `transline, another output path connecting theoutinittingboth of said frequency ranges tothe put side of said amplifier to said one line, oney filter inl the path, and line filters in-said lines frequency grouping direction filter jin said respectively,` each of said line filters passing .one Output patlnfor passing one range of freboth of said'. frequency ranges and suppressqucncies and suppressing a lower range, .aning a, frequency range lower than either. other output;` frequency grouping direction filter in said other outpujy'path, 'for passing diiectionally transmitting amplifier common Said lower range and suppressing Said .one to both directions of transmission and means range, one input filter in said one input path, rcsponsivetoA amplified waves frein said am for passing said one range and suppressing Dlificr for Controlling the transmission equivsaid other range, another input filterfin said alent- Of Seid repeater independently fOr the other input path for passing said lower WO diIeCIOIlS` i 19.' A two -way repeater comprising a uni-- range and suppressing said one range, an at-I tenuation equalizer connected between said oneline and a major portion of said one in-I put filter, another attenuation'equalizer 'and In Witness whereof,l I hereunto subscribe my name this 25th day of June, 1928.

HAROLD s. BLACK.

a transmission equivalent adjusting network i connected between said other line and a ina- )or portion of saidother input filter", said one equalizer equalizing the attenuation of said one line over said one -frequency range, said other equalizer equalizing the attenuation of said other line over said lower-frequency range, said network compensatiiigfor lower attenuation of said other line for said lower frequency range than for said one frequency range, and each of said input paths transmitting both of said frequency ranges to the filter in the path. i

' 18. A wave transmission systein comprising two lines, a one-way amplifier for repeating from one of said lines toY the other and' from said other line to said one line, `oneinput path connecting said one line to the input side of said amplifier,-another input .path connecting said other line to the input side of saidampl-ifier, one output path connectingfthe output side of said amplifier to said other line, another output path connecting the output' side ofsaid amplifier to said one line, one

frequency groupingdirection,filter in said one output path,' for passing one range of i frequencies and suppressing a lower range, another output frequency grouping direction filter in said other output path, forpassing Y said flower' range and suppressing said one range, one input filter in said one input path,

for passing said one range and suppressing' 

